Electrode binder

ABSTRACT

The present invention provides a process for the preparation of an electrode binder, which process comprises:  
     (a) thermally cracking a non-hydrotreated thermal tar feedstock having an aromatic content of less than 65 % wt;  
     (b) separating the thermally cracked product of step (a) in a separator, into at least a top fraction and a bottom fraction, and  
     (c) subjecting the bottom fraction from step (b) to vacuum distillation, to yield, as a vacuum distillation residue, an electrode binder, and an anode binder obtainable by said process for use in aluminium production.

[0001] The present invention relates to a process for preparing anelectrode binder, and an electrode binder obtainable by said process.

BACKGROUND OF THE INVENTION

[0002] Electrodes are used in smelting cells for the production ofmetals such as aluminium and steel, at present nearly all primaryaluminium being produced by electrolysis of alumina (Al₂O₃) inelectrolysis cells. In an electrolysis cell for aluminium production,aluminium is deposited in molten form onto a carbon cathode whilstsimultaneously oxygen is released at, and eventually consumes, thecell's anode. The electrodes are prepared by mixing petroleum cokeparticles with a binder. Petroleum coke comprises nearly pure carbon andis formed during the refining of crude oil by high temperaturecarbonisation of heavy residues.

[0003] Two main categories of anode are employed in aluminiumelectrolysis cells, pre-baked anodes and so-called Soederberg anodeswhich are used in Soederberg cells. In Soederberg cells, a continuousmixture of petroleum coke and binder is fed into the cell, the anodebeing baked in situ by the heat generated in the cell. Pre-baked anodesare prepared by pressing a mixture of petroleum coke particles andbinder into shape and then subjecting the anodes to baking orcarbonisation in order to transform the binder into carbon.

[0004] The binder usually used in the production of such electrodes iscoal-tar pitch. Coal-tar pitch is a distillation product of coal tar,coal-tar being a product of the carbonisation of coal, and consists ofhydrocarbon oils, and derivatives of phenols and bases such as pyridineand quinoline etc. Coal-tar pitch is used as a binder as its high carbonand aromatic content has meant that after carbonisation electrodescontaining a coal-tar pitch binder contain few non-carbon impurities.This is important to the performance and life-time of the electrode asimpurities (e.g. metals such as vanadium, nickel, etc.) in the electrodemay similarly contaminate the product and increase the air- and carbondioxide-reactivity of the electrode thus reducing its operationallife-time. However, coal-tar contains an extremely high proportion ofPolycyclic Aromatic Compounds (PAC), a typical coal-tar pitch having aPAC content of approximately 100,000 ppm. Some of these molecules arecarcinogenic and for environmental and health and safety reasons itwould be beneficial if there was an alternative material that could beused as an electrode binder in place of coal-tar pitch, but which has aslow a PAC content as possible.

[0005] The need for a coal-tar pitch replacement has been increased byrecent regulatory trends making it preferable that PAC levels incommercial materials be kept to a minimum. A large number of PACmolecules exist. In Europe certain substances (e.g. coal derivedsubstances such as coal-tar pitch) are classified according to theircontent of specific PACs. One such PAC is benzo[a]pyrene, which isconsidered to be a useful marker of the overall PAC content of asubstance. Accordingly, the classification of such products in respectof carcinogenicity is generally based upon their benzo[a]pyrene content.

[0006] The use of heavy petroleum residues as electrode binders has beeninvestigated. However, to date such binders have not been consideredindustry-acceptable replacements for coal-tar pitch as their performancewith respect to coal-tar pitch in important parameters such as theircarbon content and density, as well as the air-reactivity and carbondioxide-reactivity of electrodes prepared from such binders has beenunsatisfactory. Further, of the few reported means by which satisfactoryelectrode binders may be prepared, the petroleum residues employed asthe feedstock are highly aromatic, resulting in products having a PACcontent which remains high and which is undesirable for environmentaland health and safety reasons.

[0007] For example, EP-A 0378326 describes a binder pitch suitable foruse in the preparation of graphite electrodes used in electric arcfurnaces for the production of steel, by subjecting a petroleum aromaticmineral oil to hydrotreating; thermally cracking the hydrotreatedaromatic mineral oil; subjecting residue from the thermal cracking todistillation and combining the topped residue with finely subdividedcalcined premium coke particles having an average diameter between 1 and40 μm. In this process, which relates specifically to the preparation ofbinder pitch for steel production, it is necessary to use a hydrotreatedaromatic mineral oil feedstock i.e. a feedstock which has beenpre-treated with hydrogen in the presence of a catalyst.

[0008] Canadian Patent publication 2009121 describes a process for theproduction of a high quality petroleum tar pitch from an aromaticfeedstock comprising the steps of, (a) providing a fresh aromaticfeedstock; (b) pre-heating said feedstock in a furnace to a temperatureof about between 380 to 480° C.; (c) feeding said heated feedstock to areactor and treating said feedstock in said reactor under controlledconditions so as to promote condensation and polymerization reactions;(d) passing said treated feedstock to a fractionating tower wherein thefeedstock is fractionated into (1) gases, (2) light distillates and (3)a bottom fraction stream; (e) dividing said bottom fraction stream intoa recycle stream and a cracked fraction stream; and (f) feeding saidcracked fraction stream to a reduced pressure distillation tower whereinsaid light cracked fraction is further fractionated into (1) light gasoil, (2) heavy gas oil and (3) a high quality petroleum tar pitch.

[0009] The feedstock of the process of CA 2009121 is a highly aromatichydrocarbon stream having an aromatics content of 65-85% wt (page 4,lines 1-4 and page 6, line 21 to page 7, line 5). In the working exampleprovided, the feedstock employed is a catalytic cracking decanted(clarified) oil having an aromatic content of 85% wt. Further, in orderto obtain a binder pitch having the correct properties, it is essentialto recycle a part of the heavy cracked fraction using a recycle stream,it being stated that recycling is highly desirable in order to optimisethe resulting pitch properties (page 10, lines 1 to 5). Therefore, fromthe teaching of CA 2009121 the person skilled in the art would be led toconclude that for an electrode binder to be prepared from a petroleumresidue by thermal cracking of that residue, it is necessary to use ahighly aromatic feedstock, even requiring recycle of a part of thethermally cracked residue.

[0010] It would be advantageous if there was a means by which anelectrode binder having both industry-acceptable properties and a lowPAC content could be prepared from a petroleum residue.

SUMMARY OF THE INVENTION

[0011] It has now surprisingly been found possible to prepare from apetroleum residue an electrode binder which has properties approachingthat of coal-tar pitch and a PAC content significantly lower than thatof existing petroleum based-binders.

[0012] The present invention provides a process for preparing anelectrode binder, which process comprises:

[0013] (a) thermally cracking a non-hydrotreated thermal tar feedstockhaving an aromatic content of less than 65% wt,

[0014] (b) separating the thermally cracked product of step (a) in aseparator, into at least a top fraction and a bottom fraction, and

[0015] (c) subjecting the bottom fraction from step (b) to vacuumdistillation, to yield as a vacuum distillation residue an electrodebinder.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention involves thermally cracking anon-hydrotreated thermal tar feedstock having an aromatic content ofless than 65% wt [step (a)]. Preferably, the aromatic content of thethermal tar feedstock is less than 60% wt, more preferably less than 55%wt, and most preferably less than 50% wt, aromatic content being thetotal amount of mono to hexa+aromatic compounds as measured according totest method SMS 2783-95 (Research Disclosure 451104, November 2001, No.451, pp 1918-1922). Preferably, the thermal tar feedstock has anaromatic content in the range of from 25 to 65% wt, more preferably 30to 60% wt and most preferably 30 to 50% wt.

[0017] Thermal tar is a residual product of thermal cracking; thermalcracking being a process wherein hydrocarbons are heated to hightemperature (e.g. 400 to 500° C.) at which temperatures longerhydrocarbon molecules become unstable and break into smaller molecules.Thermal cracking feeds are generally a mixture of heavy hydrocarbonsleft over from atmospheric distillation or vacuum distillation of acrude oil, for example short residues, gas oils and heavy or waxydistillates. The major applications of thermal cracking in refineriesare in visbreaking (i.e. viscosity reduction) and thermal gas oilproduction.

[0018] The thermal tar feedstock of the present invention may be anythermally cracked residue having an aromatic content of less than 65%wt. Preferably, the thermal tar feedstock is a residue from a thermalgas oil unit.

[0019] It is particularly preferred that the thermal tar feedstock ofthe present invention is a residue from the thermal cracking of a heavydistillate feed, preferably a waxy distillate feed. Such thermal tarfeedstocks are preferred as they contain fewer metal impurities thanthermal tars obtained by cracking, for example, residual feeds. Mostpreferably the thermal tar feedstock of the present invention isobtained by thermally cracking a heavy, preferably a waxy, distillatefeed in a thermal gas oil unit.

[0020] An example of a thermal tar feedstock which has givenparticularly good results when used in accordance with the presentinvention may be obtained from a thermal gas oil unit in a processcomprising: i) thermal cracking of a residual feed, ii) separating thecracked feed into a gas fraction and a liquid fraction, iii) separatingthe liquid fraction into at least a gas oil fraction and a waxydistillate fraction in a fractionator, iv) thermally cracking the waxydistillate fraction and v) separating the thermally cracked waxydistillate fraction to yield, as a residue, a thermal tar. The thermallycracked waxy distillate fraction may be conveniently separated in thefractionator of step iii). In this example, the residual feed ispreferably a short residue, and where the fractionator of step iii) isan atmospheric fractionator the waxy distillate fraction is preferablydrawn from the fractionator at a temperature in the range of from 350 to450° C., more preferably of from 370 to 420° C.

[0021] The thermal tar feedstock of the present invention is anon-hydrotreated feedstock. By non-hydrotreated it is meant that thefeedstock has not been treated with hydrogen, as for example in acatalytic hydrotreater. Hydrotreating a feedstock lowers its carboncontent and density. This is considered disadvantageous for theproduction of electrode binders as it is preferred that electrodebinders have a high carbon content, and thus a high density.

[0022] In the present invention the thermal tar feedstock is subjectedto thermal cracking. The feedstock may conveniently be cracked in acracking furnace. When a cracking furnace is employed the feedstock ispreferably heated to a temperature in the range of from 400 to 500° C.,more preferably 450 to 500° C. The residence time may vary depending onthe feedstock and cracking temperature, however in refinery operationsit may conveniently be in the range of from 1 to 10 minutes, preferably1 to 5 minutes.

[0023] Preferably, the thermal tar feedstock is thermally cracked in avisbreaking unit. A visbreaking unit comprises a furnace and a soaker.In operation, feed is firstly heated to cracking temperature in thefurnace and then passed to the soaker where most of the cracking takesplace under pressure. When a visbreaking unit is employed the feedstockis preferably heated in the furnace to a temperature in the range offrom 400 to 500° C., more preferably 450 to 500° C. and then transferredto the soaker reactor. The pressure in the soaker reactor is preferablyin the range of from 100 kPa to 1000 kPa, more preferably 150 to 500kPa, and most preferably 200 to 300 kPa; and the residence time ispreferably in the range of from 10 to 120 minutes, more preferably 15 to60 minutes, and most preferably 20 to 40 minutes.

[0024] In the process of the present invention, thermal cracking in avisbreaking unit is particularly effective when the feedstock is athermal tar from a thermal gas oil unit.

[0025] The thermally cracked product of step (a) is separated into atleast a top fraction and a bottom fraction in a separator [step (b)].

[0026] The separator of the present invention may be any apparatuscapable of separating the thermally cracked product into a top fractionand a bottom fraction i.e. a lighter fraction comprising lightermolecules (top fraction) and a residual fraction comprising heavymolecules (bottom fraction). Examples of apparatus which may be used asthe separator include an atmospheric distillation unit, a vacuumdistillation unit and a cyclone. Regardless of the type of separatoremployed, the bottom fraction preferably comprises at least 80% wt ofcomponents having an atmospheric boiling point of at least 300° C.

[0027] Preferably, the separator of step (b) is a cyclone. When acyclone is employed the thermally cracked product of step (a) isseparated into a gas fraction (top fraction) and liquid fraction (bottomfraction). It is preferred to use a cyclone to separate the thermallycracked product of step (a) as the high viscosity of this material (i.e.a thermally cracked thermal tar) is such that when introduced intodistillation apparatus the thermally cracked product may block or evencoke in the distillation apparatus.

[0028] When a cyclone is employed the temperature of the thermallycracked product entering the cyclone is preferably in the range of from350 to 450° C., more preferably 360 to 400° C.; and the residence timein the cyclone is preferably in the range of from 5 to 30 minutes, morepreferably 8 to 20 minutes.

[0029] It is an advantageous feature of the present invention that it isnot necessary to recycle any part of the bottom fraction of separationstep (b) back through thermal cracking step (a). Accordingly, in apreferred process according to the present invention, no part of thebottom fraction of separation step (b) is recycled back through thermalcracking step (a).

[0030] The bottom fraction from the separator is subjected to vacuumdistillation, the residue of said vacuum distillation being theelectrode binder of the present invention [step (c)].

[0031] The vacuum distillation of the bottom fraction is preferablycarried out at a pressure in the range of from 0.3 to 16 kPa, morepreferably 1 to 10 kPa, even more preferably 1 to 8 kPa, and mostpreferably 3 to 7 kPa; and a distillation temperature preferably in therange of from 310 to 400° C., more preferably 320 to 390° C. and mostpreferably 350 to 380° C. The conditions of vacuum distillation arepreferably such that they correspond to an atmospheric boiling point offrom 450 to 550° C., more preferably of from 480 to 520° C., whereinconversion of atmospheric boiling point to sub atmospheric boiling pointis made in accordance with the Maxwell-Bonell relationship as describedin Ind. Eng. Chem., 49 (1957) pp 1187-1196).

[0032] As is well known to those skilled in the art, it is preferredthat the carbon content of an electrode binder be as high as practicallypossible. This is because in preparing the electrode the binder will beconverted into carbon. Accordingly, the conditions employed in thethermal cracking, separation and vacuum distillation steps arepreferably optimised such that the electrode binder has a Micro CarbonResidue Test (MCRT) value of at least 45% wt; more preferably at least50% wt (as measured according to DIN EN ISO 10370).

[0033] The electrode binder of the present invention may comprise avacuum distillation residue from a sole process stream according to thepresent invention or it may conveniently comprise a blend of two or moresuch vacuum distillation residues. Blends of two or more differentvacuum distillation residues prepared according to the present inventionmay be conveniently used to optimise the properties of the electrodebinder.

[0034] It is an advantageous feature of the present invention that theelectrode binders comprise only low levels of PAC molecules as comparedto coal-tar pitch and the hereinbefore described petroleum basedbinders. Preferably, an electrode binder according to the presentinvention has a benzo[a]pyrene content of less than 200 ppm, morepreferably less than 100 ppm, and most preferably less than 50 ppm, asmeasured according to IP BN/93.

[0035] Preferably, the electrode binders have a PAC content of less than2000 ppm, more preferably less than 1000 ppm, and most preferably lessthan 750 ppm, as measured according to IP BN/93 on the basis of the listof PAC molecules provided in respect of Examples 1-4.

[0036] It is a further preferred feature of the present invention thatthe electrode binders have a low sulphur content, the electrode binderspreferably having a sulphur content of less than 2% wt, more preferablyless than 1% wt, as measured according to ASTM 2622-94.

[0037] The present invention further provides for an electrode binderobtainable by the process of the present invention.

[0038] The present invention still further provides for an anode binderfor use in aluminium production obtainable by the process of the presentinvention; and for the use of said anode binder in aluminium production.

[0039] The invention will be further understood from the followingillustrative examples.

[0040] In the following Examples, unless otherwise stated, densityvalues were measured at 25° C. by test method DIN 52004; Micro CarbonResidue Test (MCRT) values were measured by test method DIN EN ISO10370; Sulphur content was measured by test method ASTM D 2622-94;Softening point was measured by test method DIN 52011; Viscosity valueswere measured at the specified temperature using a Dynamic ShearRheometer.

[0041] Total aromatic content was determined according to Shell MethodSeries (SMS) 2783-95 (Research Disclosure 451104, November 2001, No.451, pp 1918-1922) and is based on the total amount of mono to hexa⁺aromatic compounds present. SMS 2783-95 is a means of ultravioletquantitative analysis based on ASTM E169-99. The ultravioletspectrometer employed was a single beam instrument(Varian Cary 50),having a bandwith of 1.0 nm or less at 220 nm; a photometricrepeatability of 0.5% Transmission and a slit width of 2 nm. Thespectrophotometer was fitted with a matched stoppered silica cell ofcertified pathlength. Absorbance maxima were measured at threewavelength positions: 190 to 205 nm (λ₁): 218 to 238 nm (λ₂): and 245 to265 nm (λ₃): which positions correspond to the absorption bands ofmono-, di-, and tri-aromatic compounds respectively. Wavelengthpositions were derived for higher aromatic compounds from the peakmaxima for λ₃ as described in SMS 2783-95. Quantities of each aromatictype were calculated from the absorptivity of each absorbance maxima bycorrelating the data with those of a calibration sample of knownconcentration using the method described in SMS 2783-95, usingcalculation procedure number 1.

[0042] PAC content was determined according to IP BN/93, wherein testsamples were sequentially filtered on silica with toluene; filtered onsilica with heptane; PAC species separated from aliphatics, naphthenics,mono- and diaromatic hydrocarbons by high performance liquidchromatography (HPLC); and finally identification and quantification ofindividual PAC species performed by GC-MS. In this analysis,12Deutorated Benzo[a]pyrene was employed as an internal standard. ThePAC species measured in determining the PAC content were as follows:fluoranthene; pyrene; benzo[a]fluorene; benzo[b+c]fluorene;benzo[b]naphto[2,1-d]thiophene; benzo[g,h,i]fluoranthene;benzo[a]anthracene; chrysene and triphenylene; 1+2+3+4+5+6methylchrysene; benzo[b,j,k]fluoranthene; benzo[e]pyrene;benzo[a]pyrene; perylene; dibenz[a,j]anthracene;indeno[1,2,3-c,d]pyrene; dibenz[a,h+a,c]anthracene; benzo[b]chrysene;benzo[g,h,i]perylene; anthanthrene, and coronene.

EXAMPLES 1-4 Preparation of Electrode Binders

[0043] An electrode binder was prepared using a feedstock and processaccording to the invention (Example 1). In addition, comparative binderswere prepared using alternative types of feedstock (Examples 2-4). Thefeedstocks employed in Examples 1-4 were:

[0044] Example 1: (According to the Invention) Feedstock (A). A thermaltar from a thermal gas oil unit (aromatic content 49.5% wt).

[0045] Example 2: (Comparative) Feedstock (B). Clarified oil; a highlyaromatic oil (aromatic content 76.0% wt) which is the residue from anatmospheric fractionator on a catalytic cracking unit.

[0046] Example 3; (Comparative) Feedstock (C). A short residue from aNorth Sea crude, which is the bottom product of a vacuum distillationunit (aromatic content 30.4% wt).

[0047] Example 4: (Comparative) Feedstock (D): An ethylene crackerresidue (ECR), which is the residue from an atmospheric fractionator ofan ethylene cracker unit (aromatic content 70.7% wt).

[0048] The properties of feedstocks A to D are shown in Table 1. TABLE 1Thermal Clarified Short Tar Oil Residue ECR Feedstock (A) (B) (C) (D)Density (g/cm³) 1.029 1.119 1.018 1.137 MCRT (% wt) 12.5 13.5 21.6 25.0Sulphur (% wt) 0.72 2.50 2.20 0.23 Softening Liquid liquid 51.0 44.5Point (° C.) Viscosity 248 56 32718 5191 80° C. (mm²/s) PAC (ppm) 680020132 34 32472 Vanadium (ppm) <5 3 95 <1 Nickel (ppm) <5 3 25 <1 TotalAromatic 49.5 76.0 30.4 70.7 (% wt)

[0049] Electrode binders were prepared from feedstocks A-D as follows.

[0050] Feedstock was passed through a filter manifold and stored in aheavy oil weight tank. From the weight tank, the feedstock was pumped toa cracking furnace. The cracking furnace comprised eight separate tubecoils each immersed in a lead pot with each lead pot separately heatedby an electrical heater. The total volume of the coils was approximately3000 cm³. The residence time of the feedstock in the furnace wasdependant upon the density of the feedstock and the feed rate, however afeed rate of 5 Kg/h gave a residence time of approximately 40 minutes,and a feed rate of 2.5 Kg/h gave a residence time of approximately 80minutes.

[0051] After passing through the cracking furnace the total thermallycracked product was fed into a flash column separator wherein overheadvapours, butane and lighter fractions were removed. The bottom fractionfrom the separator (fractionator bottoms) was then fed into a vacuumtower wherein it was further separated into two fractions by vacuumflash distillation, an over-head fraction and a residue fraction, whichresidue fraction was collected for use as an electrode binder.

[0052] The yields and properties of the electrode binders obtained areshown in Table 2, together with the conditions employed in the crackingfurnace and vacuum flasher. For each Feedstock the conditions wereoptimised to obtain an electrode binder having a MCRT value of at least45% wt. In Example 1 (Feedstock (A)), two runs were performed yieldingtwo binders, (A1) and (A2). Similarly, in Example 2 (Feedstock (B)) tworuns were performed yielding binders (B1) and (B2). TABLE 2 Example 2Example 3 Example 4 Example 1 (comp.) (comp.) (comp.) Feedstock ThermalTar (A) Clarified Oil (B) Short Residue (C) ECR (D) Binder Properties A1A2 B1 B2 C1 D1 PAC (ppm) 620 1390 2050 9748 5 20000 Benzo[a]pyrene (ppm)51 120 200 590 1 1600 MCRT (% wt) 53.7 46.4 63.4 41.4 51.8 62.0 Density(g/cm³⁾ 1.150 1.142 1.203 1.222 1.094 1.213 Softening Point (° C.) 133.0110.0 159.0 97 138.5 >160 Viscosity 11818 1163 NT 118 23961 NT 180° C.(mPas) Sulphur (% wt) 0.74 0.74 2.3 2.5 2.5 0.13 Vanadium (ppm) <10 <10<10 <10 245 <10 Nickel (ppm) <10 14 26 24 81 14 Run Feed Rate (Kg/h) 2.52.5 2.5 2.5 5.0 2.5 Max Coil Temp ⁽° C.) 465 457 438 440 487 443 Vac.Tower Temp (° C.) 363 362 347 349 373 347 Vac. Tower Pressure 1.6 1.62.5 3.3 1.1 6.1 (kPa) Binder Yield (%) 29 33 25 25 40 46

[0053] From Table 2 it can be seen that the electrode binders of Example1, (A1) and (A2), prepared from a thermal tar feedstock, have a lowsulphur content and low PAC/benzo[a]pyrene content. In this regard theycompare favourably to the binders of Example 2, (B1) and (B2), preparedfrom a clarified oil feedstock, which have a high sulphur and PACcontent, and to electrode binder (D1) prepared in Example 4 from anethylene cracker residue feedstock which has an even higher PAC content.Whilst electrode binder (C1), prepared in Example 3 from a short residuefeedstock had low PAC content, the density of the binder obtained waslow.

EXAMPLES 5-9 Preparation and Testing of Laboratory Electrodes

[0054] Test electrodes were prepared using the electrode binders ofExamples 1-4.

[0055] In Example 5 (according to the invention) an electrode wasprepared from a blend of binders (A1) and (A2). Similarly, in Example 6a comparative electrode was prepared from a blend of binders (B1) and(B2). In Example 8, binder (D1) was blended with a small amount ofover-head fraction (OH) obtained from the vacuum tower as the viscosityof D1 alone was too high to prepare a laboratory scale electrode. Afurther comparative electrode was prepared using a coal-tar pitch binder(Example 9). The composition and properties of the binders used toprepare the test electrodes are shown in Table 3.

[0056] Test electrodes were prepared as follows. Graded petroleum cokewas preheated to a temperature of at least 110° C. greater than thesoftening point of the binder to be used. The petroleum coke was thenplaced in a similarly preheated mixer and cold crushed binder blendedinto the preheated petroleum coke. Mixing was continued until ahomogenous blend was obtained, after which the mixture was transferredto a preheated steel mould and the electrode shaped by means of ahydraulic press with an applied pressure of approximately 10 tonnes. Theelectrodes comprised 14% wt binder, 19.7% wt course petrol coke, 26.5%wt medium petrol coke, 15.8% wt fine petrol coke and 24% wt dust petrolcoke.

[0057] The electrodes were calcified by oven baking, applying a finaltemperature of 1190° C., for 24 hours. During the baking process, theelectrodes were protected against oxidation by a covering of petrol cokeand by flushing the oven with nitrogen. The electrodes were then testedfor air-reactivity, and carbon dioxide-reactivity according to themethods described in Fischer W. K. et al, Journal of Metals 39 (11),43-45, 1987. Electrical resistance was tested according to test methodDIN 51919. The results are shown in Table 3. TABLE 3 Example 6 Example 7Example 8 Example 9 Example 5 (comp.) (comp.) (comp.) (comp.) Binder A1(85% wt) B1 (55% wt) C1 (100% wt) D1 (87% wt) Coal-Tar A2 (15% wt) B2(45% wt) OH (13% wt) Pitch (100%) PAC (ppm) 736 5514 5 20000 97800Benzo[a]pyrene (ppm) 61 376 1 1600 10900 Viscosity 6.940 2.850 12.3817.640 1.032 180° C. (mPas) Softening Point ° C. 130.5 135.5 131 129115.5 MCRT (% wt) 52.6 54.8 50.3 53.6 54.7 Electrode Air-Reactivity 92.594.3 73.2 92.7 92.7 (% Residue) ^(a)) CO₂-Reactivity 85.2 86.4 82.3 83.582.3 (% Residue) ^(b)) Electrical Resistance 70.4 69.0 80.9 79.5 78.5(μΩm)

[0058] From Table 3 it can be seen that the electrode of Example 5,prepared according to the present invention, was prepared from a binderhaving a very low PAC/benzo[a]pyrene content, and displayed aperformance in terms of air-reactivity, carbon dioxide-reactivity andelectrical resistance similar to that of a conventional electrodeprepared from coal-tar pitch (Example 9).

[0059] The electrodes of Examples 6 and 8, were prepared from bindershaving a high PAC/benzo[a]pyrene content and their use would thereforebe undesirable from an environmental and health and safety standpoint.Whilst the electrode of Example 7 was prepared from a binder with a lowPAC content, it displayed very low air reactivity and thus the binderfrom which this electrode was prepared would not be consideredacceptable as a replacement for coal-tar pitch by the aluminium andsteel industry.

EXAMPLE 10

[0060] An electrode binder was prepared from a thermal tar taken fromthe bottom of an atmospheric fractionator of a two-stage thermal gas oilunit (TGU). The electrode binder was prepared by thermally cracking thethermal tar through a cracking furnace (temperature 480° C.), passingthe thermally cracked product through a soaker reactor (pressure 2.5 bar(250 kPa), residence time 25 to 35 minutes, temperature 450° C.),separating the thermally cracked product in a cyclone (inlet temperature390° C.) into a top and bottom fraction, (residence time 8 to 20minutes) and subjecting the bottom fraction to vacuum distillation in avacuum flasher (temperature 370° C., pressure 55 mbar (5.5 kPa). Theproperties of the thermal tar feedstock and the electrode binderobtained are shown in Table 4.

[0061] An electrode was prepared from the electrode binder of Example 10in the same manner as the electrodes of Examples 5-9 were prepared; andtested for air reactivity, carbon dioxide reactivity, and electricalresistance. The results are shown in Table 4.

[0062] From Table 4 it can be seen that the binder of Example 10 had alow PAC/benzo[a]pyrene content and the electrode prepared from thebinder had air-reactivity, carbon dioxide-reactivity and electricalresistance similar to that of an electrode prepared from coal-tar pitch.TABLE 4 Thermal Tar Electrode Example 10 Feedstock Binder Electrode PAC(ppm) 1163 622 NT Benzo[a]pyrene (ppm) 86 73 NT Density (g/cm³) 1.0011.167 NT MCRT (% wt) 11.5 51.6 NT Sulphur (% wt) 1.11 1.2 NT SofteningPoint (° C.) Liquid 127 NT Total Aromatic (% w) 34 NT NT Viscosity NT4.468 NT 180° C. (mPas) Air-Reactivity (% Residue) NT NT 91.8 (92.7)*CO₂-Reactivity (% Residue) NT NT 84.1 (82.3)* Electrical Resistance(μΩM) NT NT 79.8 (78.5)*

What is claimed is:
 1. A process for the preparation of an electrodebinder, which process comprises: (a) thermally cracking anon-hydrotreated thermal tar feedstock having an aromatic content ofless than 65% wt, (b) separating the thermally cracked product of step(a) in a separator, into at least a top fraction and a bottom fraction,and (c) subjecting the bottom fraction from step (b) to vacuumdistillation, to yield, as a vacuum distillation residue, an electrodebinder.
 2. A process as claimed in claim 1, wherein the thermal tarfeedstock is a residue from a thermal gas oil unit.
 3. A process asclaimed in claim 1, wherein the thermal tar feedstock is a residue fromthe thermal cracking of a waxy distillate feed.
 4. A process as claimedin claim 1, wherein the separator of step (b) is a cyclone.
 5. A processas claimed in claim 1, wherein the thermal tar feedstock is thermallycracked in a visbreaking unit.
 6. A process as claimed in claim 1,wherein the electrode binder has a sulphur content of less than 2% wt.7. A process as claimed in claim 1, wherein the electrode binder has aPAC content of less than 1000 ppm.
 8. A process as claimed in claim 1,wherein the electrode binder has a benzo[a]pyrene content of less than200 ppm.
 9. An electrode binder obtainable by a process as claimed inclaim
 1. 10. An anode binder obtainable by a process as claimed in claim1 for use in aluminium production.